SQL: how to limit a join on the first found row? - sql

Question

How to make a join between two tables but limiting to the first row that meets the join condition ?
In this simple example, I would like to get for every row in table_A the first row from table_B that satisfies the condition :
select table_A.id, table_A.name, table_B.city
from table_A join table_B
on table_A.id = table_B.id2
where ..
table_A (id, name)
1, John
2, Marc
table_B (id2, city)
1, New York
1, Toronto
2, Boston
The output would be:
1, John, New York
2, Marc, Boston
May be Oracle provides such a function (performance is a concern).

Answer 1

The key word here is FIRST. You can use analytic function FIRST_VALUE or aggregate construct FIRST.
For FIRST or LAST the performance is never worse and frequently better than the equivalent FIRST_VALUE or LAST_VALUE construct because we don't have a superfluous window sort and as a consequence a lower execution cost:
select table_A.id, table_A.name, firstFromB.city
from table_A
join (
select table_B.id2, max(table_B.city) keep (dense_rank first order by table_B.city) city
from table_b
group by table_B.id2
) firstFromB on firstFromB.id2 = table_A.id
where 1=1 /* some conditions here */
;
Since 12c introduced operator LATERAL, as well as CROSS/OUTER APPLY joins, make it possible to use a correlated subquery on right side of JOIN clause:
select table_A.id, table_A.name, firstFromB.city
from table_A
cross apply (
select max(table_B.city) keep (dense_rank first order by table_B.city) city
from table_b
where table_B.id2 = table_A.id
) firstFromB
where 1=1 /* some conditions here */
;

Answer 2

If you want just single value a scalar subquery can be used:
SELECT
id, name, (SELECT city FROM table_B WHERE id2 = table_A.id AND ROWNUM = 1) city
FROM
table_A

Answer 3

Query:
SELECT a.id,
a.name,
b.city
FROM table_A a
INNER JOIN
( SELECT id2,
city
FROM (
SELECT id2,
city,
ROW_NUMBER() OVER ( PARTITION BY id2 ORDER BY NULL ) rn
FROM Table_B
)
WHERE rn = 1
) b
ON ( a.id = b.id2 )
--WHERE ...
Outputs:
ID NAME CITY
---------- ---- --------
1 John New York
2 Marc Boston

Answer 4

select table_A.id, table_A.name,
FIRST_VALUE(table_B.city) IGNORE NULLS
OVER (PARTITION BY table_B.id2 ORDER BY table_B.city) AS "city"
from table_A join table_B
on table_A.id = table_B.id2
where ..

Answer 5

On Oracle12c there finally is the new cross/outer apply operator that will allow what you asked for without any workaround.
the following is an example that looks on dictionary views for just one of the (probably)many objects owned by those users having their name starting with 'SYS':
select *
from (
select USERNAME
from ALL_USERS
where USERNAME like 'SYS%'
) U
cross apply (
select OBJECT_NAME
from ALL_OBJECTS O
where O.OWNER = U.USERNAME
and ROWNUM = 1
)
On Oracle 11g and prior versions you should only use workarounds that generally full scan the second table based on IDs of the second table to get the same results, but for testing puposes you may enable the lateral operator (also available on 12c without need of enabling new stuff) and use this other one
-- Enables some new features
alter session set events '22829 trace name context forever';
select *
from (
select USERNAME
from ALL_USERS
where USERNAME like 'SYS%'
) U,
lateral (
select OBJECT_NAME
from ALL_OBJECTS O
where O.OWNER = U.USERNAME
and ROWNUM = 1
);

Answer 6

This solution uses the whole table, like in a regular join, but limits to the first row. I am posting this because for me the other solutions were not sufficient because they use one field only, or they have performance issues with large tables. I am no expert at Oracle so if someone can improve this please do so, I will be happy to use your version.
select *
from tableA A
cross apply (
select *
from (
select B.*,
ROW_NUMBER() OVER (
-- replace this by your own partition/order statement
partition by B.ITEM_ID order by B.DELIVERYDATE desc
) as ROW_NUM
from tableB B
where
A.ITEM_ID=B.ITEM_ID
)
where ROW_NUM=1
) B

Answer 7

I use the partition to separate the id2 and then just take the r_num = 1.
SELECT A.ID, A.NAME, B.CITY
FROM TABLE_A A,
(SELECT ID2, CITY,
ROW_NUMBER() OVER (PARTITION BY ID2 ORDER BY ID2) AS R_NUM
FROM TABLE_B) B
WHERE A.ID = B.ID2
AND R_NUM = 1;